Thermal detector and method

ABSTRACT

A thermal imaging system (10) for providing an image representative of an amount of thermal radiation incident to the system is provided. The system (10) includes a thermal detector (28 or 30) made from a layer of temperature sensitive material forming a first element of a signal-producing circuit (54). The first element (28 or 30) has either a resistance or capacitance value depending on its temperature. The system (10) also includes an integrated circuit substrate (32) having a second element (56, 58, 62, or 64) of the signal-producing circuit (54) complementary and electrically coupled to the first element (28 or 30). The signal-producing circuit (54) may produce an output signal having a frequency. The frequency of the output signal is monitored as representing an absolute temperature of the detector (28 or 30) so as to determine the amount of thermal energy incident to the system (10).

RELATED APPLICATIONS

This application is related to Application Ser. No. 08/182,865 filed onJan. 13, 1994, entitled Infrared Detector and Method, of the sameassignee, Attorney's Docket TI-18788 now U.S. Pat. No. 5,426,304;Application Ser. No. 08/182,268, filed on Jan. 13, 1994, entitledInfrared Detector and Method, of the same assignee, Attorney's DocketTI-17233 now U.S. Pat. No. 5,436,450; Application Ser. No. 08/281,711,filed on Jun. 26, 1994, entitled Thermal Imaging System With aMonolithic Focal Plane Array, of the same assignee, Attorney's DocketTI-18817; Application Ser. No. 08/229,497, filed on Apr. 19, 1994,entitled Self-Chopped Infrared Detector Array, of the same assignee,Attorney's Docket TI-18868 now U.S. Pat. No. 5,486,698; and ApplicationSer. No. 08/235,068, filed Apr. 29, 1994, entitled Thermal IsolationStructure for Hybrid Thermal Detectors, of the same assignee, Attorney'sDocket TI-18725 now U.S. Pat. No. 5,426,303.

RELATED APPLICATIONS

This application is related to Application Ser. No. 08/182,865 filed onJan. 13, 1994, entitled Infrared Detector and Method, of the sameassignee, Attorney's Docket TI-18788 now U.S. Pat. No. 5,426,304;Application Ser. No. 08/182,268, filed on Jan. 13, 1994, entitledInfrared Detector and Method, of the same assignee, Attorney's DocketTI-17233 now U.S. Pat. No. 5,436,450; Application Ser. No. 08/281,711,filed on Jun. 26, 1994, entitled Thermal Imaging System With aMonolithic Focal Plane Array, of the same assignee, Attorney's DocketTI-18817; Application Ser. No. 08/229,497, filed on Apr. 19, 1994,entitled Self-Chopped Infrared Detector Array, of the same assignee,Attorney's Docket TI-18868 now U.S. Pat. No. 5,486,698; and ApplicationSer. No. 08/235,068, filed Apr. 29, 1994, entitled Thermal IsolationStructure for Hybrid Thermal Detectors, of the same assignee, Attorney'sDocket TI-18725 now U.S. Pat. No. 5,426,303.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to infrared or thermal imaging systems,and more particularly, to uncooled thermal sensors that form amonolithic focal plane array.

BACKGROUND OF THE INVENTION

Infrared or thermal imaging systems typically use a plurality of thermalsensors to detect infrared radiation and produce an image capable ofbeing visualized by the human eye. Thermal imaging systems typicallydetect thermal radiance differences between various objects in a sceneand display these differences in thermal radiance as a visual image ofthe scene.

The basic components of a thermal imaging system generally includeoptics for collecting and focusing infrared radiation from a scene, aninfrared detector or focal plane array (FPA) having a plurality ofthermal sensors for converting infrared radiation to an electricalsignal, and electronics for amplifying and processing the electricalsignal into a visual display or for storage in an appropriate medium. Achopper is often necessary in thermal imaging systems to AC couple thedetector to the scene. The electronic processing portion of the thermalimaging system will typically subtract the signal generated from thedifferent chopper quadrants. An example of a thermal imaging system isdescribed in U.S. Pat. No. 4,143,269, issued to McCormack, et al.,entitled Ferroelectric Imaging System.

One type of thermal sensor includes a pyroelectric element formed frompyroelectric material that exhibits a state of electrical polarizationand capacitance dependent upon temperature changes in response toincident infrared radiation. An infrared absorber and common electrodeare disposed on one side of the pyroelectric elements. A sensor signalelectrode may be disposed on the opposite side of each pyroelectricelement. The infrared absorber and common electrode extend across thesurface of the focal plane array and are attached to each of thepyroelectric elements. Each pyroelectric element generally has its ownseparate sensor signal electrode. Each infrared detector element orthermal sensor is defined in part by the infrared absorber and commonelectrode and the respective sensor signal electrode. The electrodesconstitute capacitive plates and the pyroelectric element constitutes adielectric disposed between the capacitive plates. A large biasingvoltage across the detector decreases the capacitance of the detector.This, in turn, lowers the gain and increases the noise of the detectorby up to three to five orders of magnitude.

These previously developed thermal sensors often require a chopper inorder to obtain a reference signal. The chopper must be driven by adrive source and must be synchronized with the readings taken at thethermal sensors. This increases the complexity and cost of the thermalimaging system. These additional elements also require power, andtherefore, may result in a system that may not be suitable for long-termbattery powered operation.

SUMMARY OF THE INVENTION

In accordance with the present invention, a thermal imaging system isprovided that substantially eliminates or reduces disadvantages andproblems associated with previously developed thermal imaging systems.

One aspect of the present invention may include a thermal imaging systemfor providing an image representative of an amount of thermal radiationincident to the system. The system includes a thermal detector made froma layer of ferroelectric material forming a first element of asignal-producing circuit. The first element has either a resistance orcapacitance value depending on its temperature. The system also includesan integrated circuit substrate having a second element of thesignal-producing circuit complementary and electrically coupled to thefirst element. The signal-producing circuit may produce an output signalhaving a frequency. The frequency of the output signal is monitored asrepresenting an absolute temperature of the detector so as to determinethe amount of thermal energy incident to the system.

Another aspect of the present invention may provide a method forproducing an image representative of an amount of thermal radiationincident to a thermal imaging system. The method includes forming alayer of ferroelectric material as a thermal detector and first elementof a signal-producing circuit. The first element has a resistance orcapacitance value depending on its temperature. The method also includesforming a second element of the signal-producing circuit complementaryand electrically coupled to the first element in an integrated circuitsubstrate. The method also includes providing an output signal having afrequency from the signal-producing circuit with the frequency of theoutput signal depending on the value of the first element andrepresenting an absolute temperature of the detector. The method alsoincludes monitoring the frequency of the output signal of thesignal-producing circuit so as to detect a change in the temperature ofthe thermal detector.

Yet another aspect of the present invention may provide a thermalimaging system for providing an image representative of an amount ofthermal radiation incident to the system. The system includes a focalplane array including a plurality of thermal detectors. Each detectormay include a layer of either ferroelectric material or bolometricmaterial forming a first element of a signal-producing circuit. Thefirst element has a resistance or capacitance value that depends on itstemperature. The system also includes an integrated circuit substratehaving a plurality of second elements. Each second element is part ofone of a plurality of signal-producing circuits and is complementary andelectrically coupled to an associated first element. Eachsignal-producing circuit may produce an output signal having afrequency. The frequency of each output signal is monitored asrepresenting an absolute temperature of the detector so that the amountof thermal energy incident to the detector may be determined.

An important technical advantage of the present invention may includethe elimination of a chopper for operation. This reduces the complexityof a system incorporating concepts of the present invention, making thesystem easier to operate and maintain. Eliminating the chopper andassociated hardware also reduces the power requirements for the thermalimaging system making it more suitable for battery operation and allowsfor increased battery powered operating time. U.S. Patent ApplicationSer. No. 08/229,497, entitled Self-Chopped Infrared Detector Array, ofthe same assignee as the present application, Attorney's DocketTI-18868, also describes a system that does not require a separatechopper for operation.

Other technical advantages of the present invention may include possiblereduction in the size of the detector when the thermal detector is anelement of an oscillator circuit. A smaller detector uses lessintegrated circuit real estate. Smaller detectors also provide atechnical advantage of increased pixel density and, in turn, increasesthe sensitivity for a thermal imaging system employing the presentinvention.

Further technical advantages of the present invention includefabrication in accordance with monolithic integrated circuittechnologies that provide high performance and low manufacturing costs.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which likereference numbers indicate like features and wherein:

FIG. 1 is a block diagram of an exemplary thermal imaging system thatmay employ the present thermal sensor;

FIG. 2 is a schematic representation of an embodiment for the presentthermal sensor;

FIG. 3 shows an example of a Wien bridge oscillator circuit that may beused with one aspect of the present invention;

FIG. 4 is a schematic view in section with portions broken away showinga thermal sensor detector having a focal plane array, thermal isolationstructure, and integrated circuit substrate, incorporating an embodimentof the present invention; and

FIG. 5 is a schematic representation of an alternate configuration forthe detector of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiments of the present invention are illustrated inFIGS. 1 through 5, like numerals being used to refer to like andcorresponding parts of the various drawings.

FIG. 1 shows thermal imaging system 10 including an infrared (IR) lenssystem 12, detector assembly 14, drive and readout electronics 16, videoprocessor 18, timing and control 20, and display 22. Thermal imagingsystem 10 may also include chopper 24 that is, for example, a mechanicalchopper for interrupting the flow of infrared energy to the surface of afocal plane array in detector assembly 14. Lens system 12 may be, forexample, an infrared lens system having an object lens, correcting lens,and focusing lens for focusing thermal energy emanating from a scene(not explicitly shown) that is chopped by chopper 24 onto the focalplane array of detector assembly 14. It will be understood that varioustypes of choppers and lens systems are capable of performing the desiredoptical functions of system 10.

The focal plane array of detector assembly 14 includes a plurality ofthermal energy detectors for producing electrical signals representativeof the thermal energy impinging on each thermal detector. The electricalsignals may be processed by drive and readout electronics 16 and thensupplied to video processor 18. The focal plane array of detectorassembly 14 is more fully described later.

Videoprocessor 18 processes the signals representative of the scene intodisplay signals for display. Display 22 may be, for example, a cathoderay tube, and video processor 18 may be used for processing electricalsignals into a television format. Drive and readout electronics 16 arecoupled to the focal plane array of detector assembly 14 for processingthe electrical signals representative of the scene from detectorassembly 14. Timing control 20 guides the action of the chopper 24 whenprovided, drive and readout electronics 16, and videoprocessor 18, toselectively produce signals of the scene in a desired format for display22.

FIG. 2 is a schematic representation of focal plane array 26 containingdetectors 28 and 30. Focal plane array 26 would be suitable for use indetector assembly 14 of thermal imaging system 10 of FIG. 1. Detectors28 and 30 are coupled to integrated circuit (IC) substrate 32 by signalleads 34 and 36, respectively. IC substrate 32 includes contact pad 38coupled to signal lead 34 and contact pad 40 coupled to signal lead 36.Focal plane array 26 includes detector contact pad 41 coupling detector28 to signal lead 34 and contact pad 42 coupling detector 30 to signallead 36. Also coupled to detectors 28 is contact pad 44 receiving inputsignal 46 and coupled to detector 30 is contact pad 48 receiving inputsignal 50. Output signals from detectors 28 and 30 are provided to ICsubstrate 32 by signal leads 34 and 36 respectively, and coupled todetector 30 is contact pad 48 receiving input signal 50. Plate 52 coversthe top surface of focal plane array 26. Each detector 28 and 30 may bemade from a ferroelectric or bolometric material such as, for example,barium strontium titanate (BST), barium titanate, antimony sulfoiodide,vanadium oxide, lead titanate, and lead lanthanum zirconium titanate.

In one embodiment of the present invention, each detector 28 and 30forms a capacitive element. Continuing the example of FIG. 2, theferroelectric material of detector 28 would be the dielectric betweencapacitive plates represented by contact pads 41 and 48. Incidentinfrared radiation causes a temperature change in ferroelectric detector28. This temperature change results in a change in capacitance betweenthe contacts associated with detector 28. This change in capacitance forthe detector may be used to determine the thermal energy incident tofocal plane array 26.

In an alternative embodiment, detectors 28 and 30 provide resistiveelements made from, for example, bolometric or positive temperaturecoefficient (PTC) resistive material. In this embodiment of focal planearray 26, since a PTC material's electrical resistance is dependent uponthe material's temperature, incident infrared radiation causes atemperature change in PTC or bolometric elements 28 and 30 resulting ina change in their resistance. This change in resistance may be used tomeasure the change in thermal energy incident at focal plane array 26.It is noted that the present invention is not limited to theconfiguration of focal plane array 26 of FIG. 2. Other detectors may beused without departing from the concepts of the present invention.

In accordance with the present invention, the difference between thescene temperature and array temperature is transformed into a signal bythe value of a detector, whether capacitive or resistive. Because thevalue of the detector is very sensitive to temperature near the curiepoint, if the detector is one leg of an RC network in an oscillatorcircuit, the resonant frequency of the oscillator circuit will be adirect measure of the scene-array temperature differential. This is truewhether the detector is resistive or capacitive.

It is known that the resonant frequency of an oscillator is proportionalto 1/RC. For a ferroelectric material having a temperature slightly lessthan the curie temperature, the capacitive value of the ferroelectricmaterial (at low field) is a very strong function of temperature. Atemperature dependant detector may be made part of an oscillator circuitand the output of the oscillator circuit may be monitored. As the valueof the detector element changes, the resonant frequency of theoscillator circuit changes. By monitoring the resonant frequency of theoscillator circuit, the absolute temperature of each detector within afocal plane array may be determined. This principle may be applied withboth a capacitive detector made from ferroelectric material and aresistive detector made from bolometric material. Since the averagetemperature of the focal plane array may be measured for any time periodand compared to a previous time period's array temperature, the need fora chopper is eliminated while increasing the dynamic range of the focalplane array. This eliminates the need for chopper 24 of system 10 inFIG. 1, although chopper 24 may be included.

Frequency in radians (ω) is equal:

    ω=2πf

where f=frequency in Hz.

Therefore, ##EQU1## For example, if ω=10 MHz, C=2 pF, and R=50 KΩ, then##EQU2## for a 0.1° C. scene detection sensitivity.

FIG. 3 shows oscillator circuit 54 of the type known in the art as aWien bridge oscillator. Based on the type of material used to make theferroelectric detector, for example detectors 28 or 30 of FIG. 2, thedetector may be made either the capacitive or resistive element ofoscillator circuit 54.

Oscillator circuit 54 includes capacitor C₁ 56 coupled in parallel withresistor R₁ 58. This parallel combination of C₁ 56 and R₁ 58 are coupledto the input of amplifier 60. Coupled in series with the input ofamplifier 60 are resistor R₂ 62 and capacitor C₂ 64. The output ofoscillator circuit 54 is represented by reference number 66. Any of theresistive or capacitive elements of oscillator circuitry 54 may beembodied in a ferroelectric detector of a focal plane array, for exampledetectors 28 and 30 of focal plane array 26 of FIG. 2. By making one ofthe elements of oscillator circuit 54 a detector within a focal planearray, and by knowing the remaining values for the elements withinoscillator circuit 54, the temperature of the detector may bedetermined.

It is known for a Wien bridge oscillator that ##EQU3## By monitoring thefrequency of the signal at output 66, the temperature at a detector in afocal plane array may be determined. Using the example of FIG. 2,assuming detector 28 is embodied in C₁ 56, then the remaining elementsof oscillator circuit 54, including R₁ 58, amplifier 60, R₂ 62, andcapacitor C₂ 64, would be formed of known values in IC substrate 32.Output 66 could also be located in IC substrate 32. As the value of C₁56 changes due to the impingement of infrared energy on detector 28, thefrequency of the signal at output 66 changes. This change in outputsignal frequency may be processed in order to determine the change intemperature of detector 28 (C₁ 56) in focal plane array 26. It is notedthat the application of oscillator circuit 54 is not limited to thedetectors of focal plane array 26 being embodied in C₁ 56. Detectors 28or 30 of focal plane array 26 could also be embodied in C₂ 64 ofoscillator circuit 54 without departing from the concepts of the presentinvention.

In an alternative application of oscillator circuit 54, detectors 28 and30 of focal plane array 26 would be made resistive so that they may beused as R₁ 58 or R₂ 62. Infrared energy impinging on a resistivedetector changes the resistance value for the detector. This causes theresonant frequency of oscillator circuit 54 to change at output 66 aspreviously described. The frequency at output 66 may be monitored todetermine the temperature at focal plane array 26. It is noted that thesize of a resistive type detector in accordance with the presentinvention may be much smaller than capacitive type detectors previouslydeveloped. This helps reduce the physical size necessary to build afocal plane array incorporating concepts of the present invention andalso allows for increasing the density of detectors within the focalplane array.

It is noted that the inventive concepts of the present invention are notlimited to only Wien bridge oscillator circuits nor to the Wien bridgeoscillator circuit depicted in FIG. 3. Alternate configurations of Wienbridge circuit 54 and many other oscillator circuits may be used withoutdeparting from the inventive concepts of the present invention.

Normalization of the present detector within an RC network may berequired. Normalization will, however, be easy to accomplish because thecomplementary resistor of the RC network (when the detector iscapacitive) in the IC substrate may be set to match the measuredcapacitance of the detector. This may be accomplished by adjusting thelevel of resistance by laser cutting thick film resistors on the ICsubstrate prior to bonding the detector and the IC substrate with oneanother. Alternatively, normalization may be achieved by programming atransistor network on the IC substrate.

The monitoring of the output frequency from an oscillator circuitincorporating a ferroelectric temperature sensitive detector may beaccomplished by many techniques. One approach would be to multiply thecarrier wave coming out of the oscillator by a reference signal. Thereference signal should be chosen so that the frequency difference isgreater than 50 KHz. For example, with

    V.sub.REF =V.sub.o cos (9.8×10.sup.6 *t); and

    V.sub.SIG =V.sub.o cos (ω); then

    V.sub.OUT =V.sub.o /2* [cos (ω+9.8×10.sup.6) t+cos (ω-9.8×10.sup.6) t]

The first term of V_(OUT) is of such a high frequency that a low passfilter will screen it out. Because V_(OUT) may be approximately 1 volt,the second term will not need a preamplifier under each pixel. Thesecond term may then be digitized for digital signal processing.

FIG. 4 is a cross-sectional view of a possible structure for focal planearray 26 and IC Substrate 32 of FIG. 2, collectively referred tohereinafter as thermal imaging system 68. Some of the principalcomponents or structures that comprise thermal imaging system 68 includefocal plane array 26, thermal isolation structure 70, and integratedcircuit substrate 32. Focal plane array 26 comprises a plurality ofthermal detectors, including detectors 28 and 30. The quantity andlocation of thermal detectors will depend upon the desired row andcolumn configuration for focal plane array 26.

Thermal isolation structure 70 is used to provide mechanical supportduring bonding of focal plane array 26 with integrated circuit substrate32 and to thermally insulate focal plane array 26 from integratedcircuit substrate 32. For some embodiments of the present invention,thermal isolation structure 70 may be used to support focal plane array26 during formation of separate infrared absorber assembly 72. Also,thermal isolation structure 70 provides an electrical interface betweenthermal detectors 28 and 30 in focal plane array 26 and IC substrate 32.This electrical interface allows the detectors of focal plane array 26to be part of an oscillator circuit embedded in IC substrate 32 inaccordance with the concepts of the present invention.

The components of focal plane array 26 include a plurality of thermaldetectors, e.g., 28 and 30, and infrared absorber assembly 72. Detector28 is coupled to signal contact pad 41 and contact pad 44. Detector 30is coupled to signal contact pad 42 and contact pad 48. It is noted thatcontact pads 44 and 48 may be embodied in a single conductive layer thatcontacts all the detectors within focal plane array 26. One side of bothdetectors 28 and 30 may contact infrared absorber assembly 72.

Incident infrared radiation will interact with infrared absorberassembly 72 and produce a temperature change in the attached elements 28and 30. The temperature change will vary the capacitance (or resistance)of the ferroelectric elements 28 and 30. The representative outputsignals at (V_(o)) contact pads 38 or 40 will depend upon thecapacitance (or resistance) of the associated elements 28 and 30, which,in turn, is a function of the incident infrared radiation. Outputsignals at contact pads 38 and 40 from detectors 28 and 30 respectively,may then be used in an oscillator circuit embedded in IC substrate 32 sothat a change in capacitance (or resistance) in a detector may be usedto monitor temperatures within focal plane array 26.

Ferroelectric elements 28 and 30 of focal plane array 26 are isolatedthermally from one another and from IC substrate 32 to insure that thecapacitance (or resistance) associated with each thermal detectoraccurately represents the incident infrared radiation. Each thermaldetector in focal plane array 26 is individually coupled to IC substrate32 by a mesa-type structure provided by thermal isolation structure 70.Detector 28 is coupled to mesa-type structure 74 and detector 30 iscoupled to mesa-type structure 76. Each thermal detector is preferablycoupled electrically through its associated mesa-type structures tocorresponding contact pads on IC substrate 32.

The size of each mesa-type structure 74 and 76 will be dictatedprimarily by structural and thermal capacitance (or resistance)considerations. Signal leads 34 and 36 of FIG. 2 are embodied in metalstrips 78 and 80 respectively along the side of their associatedmesa-type structure 74 and 76.

Infrared absorber assembly 72 preferably comprises a layer of infraredabsorber or optical coating 82 formed from infrared absorbing materialand plate 52. For some applications, layer 82 may include multiplelayers of infrared sensitive material depending upon the specificwavelength or wavelengths of infrared radiation that thermal imagingsystem 68 is designed to detect. Plate 52 may perform several importantfunctions such as increasing the interaction of incident infraredradiation with optical coating 82. Also, plate 52 preferably has lowthermal conductivity to prevent rapid transfer of heat energy betweenferroelectric elements 28 and 30.

For one embodiment of the present invention, plate 52 may be formed frommetal such as NiCr, which has both low thermal and good electricalconductivity for this application and cooperates with optical coating 82to enhance the absorption of incident infrared radiation. For otherembodiments of the present invention, materials other than metal thathave the desired characteristics may be used to form plate 52. Thepresent invention is not limited to use with only metal plates 52.

Metallic bonding material 84 is preferably provided on signal leadelectrodes 41 and 42 to form a bump bond with similar metallic bondingmaterial 86 on each associated mesa-type structure. For someapplications conductive epoxy bonding may be satisfactorily used tomount thermal detectors 28 and 30 on their associated mesa-typestructures 74 and 76.

For thermal detectors 28 and 30, thermal (infrared) radiation incidentto focal plane array 26 is absorbed by the respective infrared absorberor optical coating 82 and transmitted as heat through plate 52 into theadjacent elements 28 and 30. The resulting temperature change inelements 28 and 30 causes a change in the capacitance (or resistance) inthe detectors. The change in capacitance (or resistance) causes a changein the resonant frequency of an oscillator circuit containing detector28 or 30. The change in frequency may be used to determine thetemperature of the detectors within focal plane array 26.

Integrated circuit substrate 32 includes the remaining elements of theoscillator circuit used in accordance with the present invention.Integrated circuit substrate 32 is bonded to focal plane array 26, withcontact pads 38 and 40 being electrically coupled to their correspondingsignal mesa strip conductors 78 and 80. Thermal isolation structure 70prevents IC substrate 32 from acting as a heat sink for the thermalenergy stored in ferroelectric detectors 28 and 30 and adverselyaffecting the capacitance (or resistance) of the detectors.

Mesa strip conductors 78 and 80 provide a signal path between the top ofeach mesa-type structure and the adjacent contact pad. For example, mesastrip conductor 78 provides an electrical path from contact pad 38 tothe top of mesa-type structure 74. Recommended materials for mesa stripconductors 78 and 80 include titanium and tungsten alloys because oftheir relatively low thermal conductivity and ease of application.

Indium bump bonding techniques may be satisfactorily used to form metalbonds between focal plane array 26 and thermal isolation structure 70.The configurations of mesa-type structures 74 and 76 and the associatedmesa strip conductors 78 and 80 are design choices, largely dependentupon thermal isolation and structural rigidity considerations.Alternative configurations for mesa-type structures 74 and 76 includemesas with sloping sidewalls and mesas with vertical sidewalls. Forsloped sidewall mesa-type structures 74 and 76, a mesa stripconfiguration for conductors 78 and 80 is recommended. For a verticalsidewall mesa, a mesa-contour configuration as shown in U.S. Pat. No.5,047,644, entitled Polyimide Thermal Isolation Mesa For a ThermalImaging System, may be more appropriate. These configurations areexemplary only, and other configurations for both the mesa-typestructures 74 and 76 and their associated conductors 78 and 80 may beused. In particular, while mesa-type structures 74 and 76 are shown assymmetrical in horizontal and vertical cross section, such symmetry isnot required.

Mesa-type structures 74 and 76 including the exemplary thermal isolationstructure 70 for thermal imaging systems 68 of FIG. 4 may be fabricatedusing conventional photolithographic techniques. Fabrication methodsusing photosensitive polyimide are preferred. However, for someapplications, non-photosensitive polyimide may be used. Fabricationusing photosensitive polyimide is recommended, because it generallyrequires fewer process steps.

One method to fabricate mesa-type structures 74 and 76 is to usephotosensitive polyimide, forming the mesa structures by patterning alayer of photoresist on polyimide, and then developing the polyimide toremove the unexposed portions. This leaves the polyimide portion of themesa structure with the desired configuration and array. The mesaconductors may then be formed with conventional metal depositionprocedures on the exterior of the polyimide structure.

Once the array of mesa-type structures 74 and 76 have been defined,selected mesa strip conductors 78 and 80 may be formed usingconventional photolithography techniques on the exterior of therespective mesas 74 and 76. Mesa strip conductors 78 and 80 arepreferably formed on the exterior of their respective mesa-typestructures 74 and 76 to extend from the top of their respectivemesa-type structure to the respective contact pads 38 and 40.

Additional fabrication steps may be employed to deposit bump-bond metals84 and 86 or conductive epoxies (not shown) on the top of mesa-typestructures 74 and 76 as desired. These additional fabrication steps areaccomplished with conventional materials and methods. The materialselection depends upon the specific application for thermal isolationstructure 70. Methods for forming thermal isolation structure 70 aredescribed in U.S. Patent Application Ser. No. (Attorney Docket No.TI-18788 (32350-0901)), filed Jan. 13, 1994, entitled Infrared Detectorand Method and U.S. Pat. No. 5,264,326, issued to Meissner, et al.,entitled Polyimide Thermal Isolation Mesa for a Thermal Imaging System.

FIG. 5 shows thermal imaging system 98 containing an alternateconfiguration for the present invention. Thermal imaging system 98includes focal plane array 100 and IC substrate 102. Focal plane array100 includes detector 104 and detector 106. Coupled to detector 104 arecontact pads 108 and 110 that are respectively coupled to signal leads112 and 114. Signal lead 112 is coupled to IC substrate 102 by contactpad 116, and signal lead 114 is coupled to IC substrate 102 by contactpad 118. Coupled to detector 106 are contact pads 120 and 122 that arecoupled to signal leads 124 and 126, respectively. Signal lead 124 iscoupled to IC substrate 102 by contact pad 128, and signal lead 126 iscoupled to IC substrate 102 by contact pad 130. Detectors 104 and 106are covered by plate 132.

Thermal imaging system 98 may be formed in accordance with the structureand processing techniques described for thermal imaging system 68 ofFIG. 4. Each signal lead of thermal imaging system 98 could be providedto IC substrate 102 by a separate mesa-type structure.

Focal plane array 100 of FIG. 5 shows an alternate embodiment where thecontact pads to the detectors are coupled on the sides of the detectorsas opposed to the top and bottom surfaces as shown in FIGS. 2 and 4. Byplacing the contact pads on the sides of detectors 104 and 106 of focalplane array 100, the size of the detectors may be reduced so that theirdensity may be increased within focal plane array 100. Increasing thedensity of the detectors within a focal plane array, in turn, increasesthe density of pixels within the focal plane array and the sensitivityof the thermal imaging system.

In operation of the present invention, by incorporating a ferroelectricthermal detector into an oscillator circuit, the frequency of theoscillator circuit varies as the value of the detector capacitance (orresistance) varies through the impingement of infrared energy andtemperature change. The detector may be resistive or capacitive withinan RC network of the oscillator circuit. The frequency of the oscillatorcircuit may be monitored so that the temperature of the detector withinthe focal plane array may be constantly determined.

The present invention provides technical advantages of eliminating theneed for a chopper in order to take temperature measurements in athermal imaging system. The present invention also allows for achievinggreater density, lower power consumption, and reduced cost overpreviously developed thermal imaging systems.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations may be made hereto without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. A method for producing an image representative ofan amount of thermal radiation incident to a thermal imaging system, themethod comprising the steps of:forming a layer of temperature sensitivematerial as a thermal detector and first element of a signal-producingcircuit, said first element having one of a resistance and capacitancevalue depending on its temperature; forming a second element of thesignal-producing circuit complementary and electrically coupled to thefirst element in an integrated circuit substrate: providing an outputsignal having a frequency from the signal-producing circuit, thefrequency of the output signal depending on the value of the firstelement and representing a temperature of the detector; and monitoringsaid frequency of said output signal of the signal-producing circuit soas to detect a change in said temperature of said thermal detector,wherein said first and second elements further comprise aresistor-capacitor (RC) network of an oscillator circuit.
 2. The methodof claim 1 wherein the first element is capacitive and the secondelement is resistive.
 3. The method of claim 1 wherein the first elementis resistive and the second element is capacitive.
 4. The method ofclaim 1 wherein the oscillator circuit further comprises a Wien bridgeoscillator circuit.
 5. The method of claim 1 further comprising the stepof forming a support structure for providing thermal and electricalisolation between the thermal detector and the substrate.
 6. The methodof claim 1 wherein the layer of temperature sensitive material furthercomprises barium strontium titanate.
 7. The method of claim 1 whereinthe temperature sensitive material further comprises a positivetemperature coefficient (PTC) resistive material.
 8. The method of claim1 wherein the layer of temperature sensitive material comprisesferroelectric material.
 9. The method of claim 1 wherein the layer oftemperature sensitive material comprises one of barium strontiumtitanate, barium titanate, antimony sulfoiodide, vanadium oxide, leadtitanate, and lead lanthanum zirconium titanate.
 10. The method of claim1 further comprising the step of forming an infrared absorber assemblyextending outwardly from the thermal detector for interacting withinfrared energy incident thereto and for producing a temperature changein the thermal detector.
 11. The method of claim 1 wherein themonitoring step further comprises detecting a change in the frequency ofthe output signal.
 12. A thermal imaging system for providing an imagerepresentative of an amount of thermal radiation incident to the system,said system comprising:a focal plane array comprising a plurality ofthermal detectors, each detector comprising a layer of temperaturesensitive material forming a first element of a signal-producingcircuit, said first element having one of a resistance and capacitancevalue depending on its temperature; an integrated circuit substratecomprising a plurality of second elements, each of said second elementspart of one of a plurality of signal-producing circuits andcomplementary and electrically coupled to an associated first element,each signal-producing circuit operable to produce an output signal at afrequency; and wherein the frequency of each output signal ismonitorable as representing an absolute temperature of the detector soas to determine the amount of thermal energy incident to the detector,wherein each associated first and second element further comprise aresistor-capacitor (RC) network of an oscillator circuit.
 13. The systemof claim 12 further comprising a support structure providing thermal andelectrical isolation between the plurality of thermal detectors and thesubstrate.
 14. The system of claim 12 wherein the layer of temperaturesensitive material comprises one of barium strontium titanate, bariumtitanate, antimony sulfoiodide, vanadium oxide, lead titanate, and leadlanthanum zirconium titanate.
 15. The system of claim 12 furthercomprising an infrared absorber assembly extending outwardly from thethermal detectors for interacting with infrared energy incident theretoand for producing a temperature change in the thermal detectors.
 16. Athermal imaging system for providing an image representative of thermalradiation incident to the system, said system comprising:an array ofthermal detectors, each detector comprising a layer of temperaturesensitive material forming a first element of a signal-producingcircuit, said first element having at least one of a resistance andcapacitance value that depends on its temperature; an array of saidsignal-producing circuits, each circuit comprising a second elementcomplementary and electrically coupled to an associated first element,said signal-producing circuit operable to produce an output signal at afrequency; and wherein the frequency of the output signal is monitoredas representing an absolute temperature of the detector so as todetermine the amount of thermal energy incident to the system, whereinsaid first and second elements further comprise a resistor-capacitor(RC) network of an oscillator circuit.
 17. The system of claim 16wherein the first element is capacitive and the second element isresistive.
 18. The system of claim 16 wherein the first element isresistive and the second element is capacitive.
 19. The system of claim12 wherein the signal-producing circuit further comprises a Wien bridgeoscillator circuit.
 20. The system of claim 16 further comprising asupport structure providing thermal and electrical isolation between thethermal detector and the substrate.
 21. The system of claim 16 whereinfirst and second electrical contacts couple to a top and bottom surfaceof the layer of temperature sensitive material.
 22. The system of claim16 wherein the first and second electrical contacts couple to a firstand second side surface of the layer of the temperature sensitivematerial.
 23. The system of claim 16 wherein the layer of temperaturesensitive material further comprises barium strontium titanate.
 24. Thesystem of claim 16 wherein the temperature sensitive material furthercomprises a positive temperature coefficient (PTC) resistive material.25. The system of claim 16 wherein the temperature sensitive materialfurther comprises ferroelectric material.
 26. The system of claim 16wherein the layer of temperature sensitive material comprises one ofbarium strontium titanate, barium titanate, antimony sulfoiodide,vanadium oxide, lead titanate, and lead lanthanum zirconium titanate.27. The system of claim 16 further comprising an infrared absorberassembly extending outwardly from the thermal detector for interactingwith infrared energy incident thereto and for producing a temperaturechange in the thermal detector.